There is a rapidly increasing demand for duplexers because of rapid spread of radio communication devices, which are typically portable telephones. Specifically, there is an intense demand for duplexers that have a compact size and a high sharpness and use acoustic wave elements.
Recently, the sophistication of radio communication systems has been developed rapidly, and the required specifications of radio frequency filters have been very complicated. For example, it is preferable that the transmission filter and the reception filter included in the duplexer have low insertion loss in the pass bands, and have high suppression in the other-filter's band (the band of the reception filter with respect to the band of the transmission filter, and the band of the transmission filter with respect to the band of the reception filter).
Generally, the filter mounted in the portable telephone terminals or the like is formed by connecting resonators together over a large number of stages in order to ensure the broad band. For example, exemplary ladder filters are disclosed in Japanese Patent Application Publication Nos. 5-167388 and 10-93375.
However, the above ladder filters have a difficulty in realizing an attenuation pole having a sufficient sharpness.
According to an aspect of the present invention, there is provided a filter having at least one series resonator and parallel resonators, the at least one series resonator and the parallel resonators including excitation electrodes and reflectors, the parallel resonators having different resonance frequencies, and at least one of the parallel resonators other than the parallel resonator having the highest resonance frequency being configured to have a pitch of reflectors that is smaller than that of excitation electrodes.